Experiments using intracellular and extracellular recording techniques have been carried out on various in vitro preparations of vertebrate and invertebrate neurons. The research has focused primarily on characterizing the types of excitable membrane processes resident in neurons and secondarily on studying the effects of various transmitter substances and clinically important drugs on these processes. The principal conclusions are that multiple forms of electrically and chemically excitable conductance mechanisms are present in spinal, hippocampal, and hypothalamic nerve cells grown in tissue culture and that both endogenous transmitters and exogenous drugs alter these conductances in superficially similar ways. The results of this research improve our basic understanding of neuronal excitability and of the physiological roles of transmitters and the pharmacological actions of drugs.